I did a few duels in the dalaran sewers and saw my friend at 1 health ramming a moonking with 1 health too..
I don't know what is up with that but I dueled my friend hunter after and he could hit me, down me to 1hp but then I got invincible.
I wouldn't end the duel. I wasn't able to hit him though.
Also was able to start a duel with a Horde, just couldn't hit him (Must be because dal is a sanctuary) I thought it was funny being able to duel your friend for 5 hours :P